A specific binding site for angiotensin II(3-8), angiotensin IV, in rabbit cardiac fibroblasts

This study demonstrates the existence of a high affinity binding site on rabbit cardiac fibroblasts of the hexapeptide (3-8) fragment of angiotensin II (AngIV). [125I]-AngIV binding is saturable, reversible and distinct from angiotensin II (AngII) receptors. At 37 degrees C equilibrium of [125I]-AngIV binding is reached within 2 h. AngIV displaces [125I]-AngIV bound to cultured rabbit cardiac fibroblasts whereas AngII receptor-specific ligands ([Sar1, Ile8]-AngII, Dup753, CGP42112A) do not. Scatchard plot analysis revealed that [125I]-AngIV binds to a single class of sites with Kd = 4.87 +/- 0.11 x 10(-9) mol/l, Bmax = 371 +/- 8.3 fmol/mg protein and a Hill coefficient of 0.92. In the presence of the non-hydrolyzable GTP analog GTPgammaS [125I]-AngIV binding in rabbit cardiac fibroblasts was not markedly affected, whereas binding of [125I]-(Sar1,IIe8)-AngII is reduced. The role of AngIV in the heart and in particular in cardiac fibroblasts is unknown, and the putative interaction of AngIV with AngII needs further characterization.